Implantable neurostimulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neurostimulation systems typically include one or more electrode carrying stimulation leads, which are implanted at the desired stimulation site, and a neurostimulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the stimulation lead(s) or indirectly to the stimulation lead(s) via a lead extension. The neurostimulation system may further comprise an external control device to remotely instruct the neurostimulator to generate electrical stimulation pulses in accordance with selected stimulation parameters.
Electrical stimulation energy may be delivered from the neurostimulator to the electrodes in the form of an pulsed electrical waveform. Thus, stimulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, duration, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “stimulation parameter set.”
With some neurostimulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neurostimulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations).
As briefly discussed above, an external control device can be used to instruct the neurostimulator to generate electrical stimulation pulses in accordance with the selected stimulation parameters. Typically, the stimulation parameters programmed into the neurostimulator can be adjusted by manipulating controls on the external control device to modify the electrical stimulation provided by the neurostimulator system to the patient. However, the number of electrodes available combined with the ability to generate a variety of complex stimulation pulses, presents a vast selection of stimulation parameter sets to the clinician or patient.
To facilitate such selection, the clinician generally programs the neurostimulator through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neurostimulator to allow the optimum stimulation parameters to be determined based on patient feedback or other means and to subsequently program the neurostimulator with the optimum stimulation parameter set or sets, which will typically be those that stimulate all of the target tissue in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
Often, multiple timing channels are used when applying electrical stimulation to target different tissue regions in a patient. For example, in the context of SCS, the patient may simultaneously experience pain in different regions (such as the lower back, left arm, and right leg) that would require the electrical stimulation of different spinal cord tissue regions. In the context of DBS, a multitude of brain structures may need to be electrically stimulated in order to simultaneously treat ailments associated with these brain structures. Each timing channel identifies the combination of electrodes used to deliver electrical pulses to the targeted tissue, as well as the characteristics of the current (pulse amplitude, pulse duration, pulse frequency, etc.) flowing through the electrodes.
The use of multiple timing channels can often lead to problems with the electrical stimulation systems due to the potential of an overlap in pulses between two or more timing channels. Overlapping of pulses using a common electrode can make neurostimulation systems ineffective or even harmful. Current neurostimulation systems employing multiple timing channels use a method known as the “token” method to prevent overlap of pulses. This method allows an electrical pulse to be transmitted in the timing channel with the “token,” while the other timing channels wait their turn. Then, the “token” is passed to the next timing channel. However, if the frequencies of the channels overlap, such that they need the “token” at the same time, transmission of an electrical pulse within the second channel must wait until the end of the transmission of the electrical pulse in the first timing channel. One possible result is that the frequency of the electrical pulses transmitted in the second timing channel gets “locked” to (i.e. matches) the frequency of the electrical pulses transmitted in the first timing channel; alternatively, one can get galloping or clumping of electrical pulses. Therefore, when the occurrence of stimulation pulses is pushed out in time, stimulation therapy becomes ineffective or even harmful for tissue regions, such as brain structures to be stimulated in DBS applications, that require stimulation at specific, regular frequencies.
The “token” method may best be understood with reference to FIG. 1. As there shown, a first pulsed electrical waveform 5a having a first frequency is transmitted within timing channel A, and a second pulsed electrical waveform 5b having a second frequency is desired to be transmitted within timing channel B. Because timing channel A has the “token,” the pulses of the second pulsed electrical waveform 5b that are to be transmitted in timing channel B must be “bumped” each time they overlap with the pulses of the first pulsed electrical waveform 5a. As can be seen in the bumped pulsed electrical waveform 5c, when a pulse is bumped (shown by the horizontal arrows), the next pulse relies on the new (bumped) pulse for timing. Thus, the next pulse is “double bumped”: once when the previous pulse is bumped and a second time when it overlaps a pulse of the pulsed electrical waveform 5a transmitted in the timing channel A. As a result, the frequency of the pulses in the second pulsed electrical waveform 5b is forced (i.e., locked) into the frequency for the first pulsed electrical waveform 5a, resulting in a pulsed electrical waveform 5d that has a frequency twice as small as the desired frequency.
There, thus, remains a need to provide an improved method for preventing or minimizing frequency locking within multi-channel neurostimulation systems.